1. Field of the Invention
The present invention relates to a power supply circuit, and in particular to a power supply circuit which switches between an output of a switching regulator and an output of a series regulator, and supplies the selected output to a load.
2. Description of the Related Art
A battery is used as a power supply of a portable electronic device such as a portable phone, a PHS (Personal Handy phone System), a PDA (Personal Digital Assistant), and a PC (Personal Computer). In order to generate a predetermined voltage to be supplied to the constituent elements of the electric device, a series regulator is used. In addition to the series regulator, a switching regulator is also used.
The series regulator can be formed using a feedback amplifier having a reference voltage input terminal to which a reference voltage is input, a feedback input terminal to which an output is input through a feedback gain, and an output terminal section.
As an example structure of the switching regulator, for example, JP 2005-174264 A discloses a voltage boosting switching regulator of a chopper type as a circuit for boosting a reduced power supply voltage to a predetermined output voltage so that a portable device can be driven even when a power supply voltage supplied from a battery equipped in the portable device having a communication function is reduced. In this reference, an example configuration is disclosed comprising an error amplifier, which compares the output voltage of the switching regulator to the reference voltage and outputs an error signal corresponding to the difference between the output voltage and the reference voltage, a PWM (pulse-width modulation) circuit which sets a duty ratio of a PWM signal based on the error signal from the error amplifier, a switching transistor which is switched ON when the PWM signal is at the high level, a voltage boosting coil having an amount of flowing current controlled in accordance with switching control of the switching transistor, and a capacitor which holds the voltage from the voltage boosting coil and outputs an output voltage.
As the distinguishing usage between the series regulator and the switching regulator, JP 2006-54969 A discloses that a method is often used in which the switching regulator is used when the power consumption of the CPU is high and the series regulator is used in a stand-by state in which the power consumption is low, in order to improve the power usage efficiency when a voltage is reduced from the battery and supplied to the CPU in the portable electronic device.
When the series regulator and the switching regulator are compared, it can be seen that the latter has a better conversion efficiency when the load is heavy, that is, when a greater amount of output current is necessary. Therefore, a method is employed as described in JP 2006-54969A where both the series regulator and the switching regulator are equipped in the portable electronic device, and the series regulator and the switching regulator are switched so that the output of the latter is used when the load is heavy, and the output of the former is used during light load.
In order to set the output voltage of the switching regulator to be in a predetermined range, feedback control is applied in which the output voltage and the reference voltage are compared using the error amplifier. In this case, because the switching regulator uses the PWM technique, if the pulse width does not match due to deficiency in phase compensation or the like, the operation of the switching regulator becomes unstable. In particular, when the switching regulator is switched from the OFF state to the ON state, the pulse generation is in transition, and the feedback of the output voltage to the error amplifier may not be sufficient. Because of this, when the output is switched from the series regulator to the switching regulator, noise may be caused.